The 3rd Battalion, 5th Marine Regiment returned home from one of Afghanistan's deadliest war zones this week after a grueling eight-month deployment with record casualties. Remarkably, military psychiatrists say the men appear to be relatively unscathed mentally.

"So far so good," said their second-in-command, Maj. Mark Carlton, who endured the 20-hour flight back with the first wave of Marines and Navy personnel from Afghanistan's Helmand Province to California's Camp Pendleton.

The battalion witnessed 25 dead, 140 wounded and more than a dozen amputees. But overall rates of combat stress among the 250 mostly infantrymen, at least in their first medical evaluations, appeared to be no higher than other units in the southern province, experts said.

Some wonder why that battalion -- nearly 1,000 in all in the heart of the Taliban insurgency -- appears so psychologically intact, when some reports show as many 37 percent of recent war veterans are being treated for post-traumatic stress disorder or PTSD.

Carlton attributed much of the good mental health to the battalion's "proactive" small-unit leadership structure.

"They know each other and live with each other the entire deployment and are never far from someone on the team," he said. "If there's a change in behavior or signs of stress, it's immediately picked up by someone who knows the guy really well."

"You absolutely see that in a lot of places and not just the military," he said. "On high school sports teams, kids get tight over time. Common understanding can't be replicated."

The battalion faced combat almost immediately when they took control of the Sangin District from the British last September. One of the fatalities was 2nd Lt. Robert Kelly, son of Lt. Gen. John Kelly, the personal military aide to Defense Secretary William Gates, the most senior officer to lose a child since American troops arrived in the country in 2001.

But as casualties mounted, visiting mental health professionals said they didn't see a comparable rise in mental health issues and were surprised by the unit's resiliency.

Now, back at Camp Pendleton, the Marines have ordered the unit to stay intact with their families for three months to allow them to decompress together. There, additional mental health professionals have been brought in to watch for signs of post-traumatic stress disorder.

An estimated 1 in 5 combat veterans will eventually be diagnosed with PTSD and 1 in 3 will have some emotional or neurological problems related to war, according to a New York University study of 300,000 returning soldiers from Iraq and Afghanistan at veterans' hospitals.

"The majority of people are highly resilient," said Dr. Charles Marmar, chair of the psychology department at NYU's Langone School of Medicine and a psychiatrist who has studied PTSD among veterans since the Vietnam War.

Post-Traumatic Stress Disorder Can Start Years Later

He said unit cohesion, proper training and a healthy personal life are all protectors against PTSD.

"In the military, people subordinate their identity for the well-being of the group," he said. "If the group is functioning well, they are more protected. If the group is in conflict, they are more vulnerable."

PTSD was first known as "soldier's heart" during the Civil War. Later, in World War I, it was called "shell shock."